$ ps -ef | rg swift
501 9986 1 0 4Sep20 ?? 0:00.00 (repl_swift)
501 10019 1 0 4Sep20 ?? 0:00.00 (repl_swift)
501 12537 1 0 4Sep20 ?? 0:00.00 (repl_swift)
501 12420 1 0 4Sep20 ttys003 0:00.00 (repl_swift)
501 13317 1 0 4Sep20 ttys006 0:00.00 (repl_swift)
501 13466 1 0 4Sep20 ttys006 0:00.00 (repl_swift)
501 13481 1 0 4Sep20 ttys006 0:00.00 (repl_swift)
...
$ ps -xo pid,ppid,stat,command | rg swift
9986 1 Z (repl_swift)
10019 1 Z (repl_swift)
12537 1 Z (repl_swift)
12420 1 Z (repl_swift)
13317 1 Z (repl_swift)
13466 1 Z (repl_swift)
13481 1 Z (repl_swift)
...
$ ps -p 1
PID TTY TIME CMD
1 ?? 12:33.42 /sbin/launchd
https://superuser.com/questions/390552/how-to-kill-a-zombie-process-on-a-mac
Sadly, it seems you have to live with zombies.